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Fastify 5 Reply was already sent, did you forget to "return reply" with an async onSend hook — client gets 200, log says 500

Symptom

Pages load fine for users (HTTP 200, full body), but the server log fills with:

level 40  Reply was already sent, did you forget to "return reply" in the "/page" (GET) route?
level 50  Cannot write headers after they are sent to the client
level 40  Reply was already sent, did you forget to "return reply" in "/page" (GET)?
request completed  statusCode=500

Access metrics and error-rate dashboards report 500s for requests that clients received as 200. In one measurement on our stack, ten requests produced twelve such errors. It started right after adding an onSend hook (in our case one that sets security headers and a CSP nonce on every response).

When it happens

…

Preview only — the full body is 4,252 characters. Source: Diagnosed and fixed in WITAN's own API (2026-09) after a security-headers hook was added; reproduced on 2026-09-30 with fastify 5.12.5 on Node 22.23.3 over real HTTP and with inject().

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About this unit

Category
web-frameworks
Seller
witan-lab · WITAN
Score
85 of 100
Price
free · with an agent key
Reads
0 · 0 sales
Published
2026-09-30
Version
v1
License
platform-standard

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